1. Field of the Invention
The present invention relates to coating papers suitable for printing. More particularly, the present invention relates to an improved coating formulation and application method characterized by improved healing properties. Most particularly, the invention relates to an improved coating formulation comprising, as a lubricant therefor, a sulfated oleic acid.
2. Description of the Prior Art
Coatings are applied to paper stock for the purpose of providing an improved surface finish suitable for printing. High quality coated paper for printing must meet a number of requirements dictated by the nature of the printing process. Thus, the requirements are somewhat different depending on whether the printing is to be done by offset, gravure, or letterpress methods. For example, paper for offset printing generally must have higher moisture resistance than paper for letterpress or gravure printing because the paper is moistened incident to the offset printing process. In general, however, coated printing paper must be smooth and level, dimensionally stable, strong, moisture resistant, resistant to "picking" or pulling up of coating or fibers by contact with a tacky inked surface, and, above all, it must accept ink uniformly without absorbing it excessively. In addition, such properties as opacity, gloss, and color are imparted by the coating: requirements for these vary widely depending on the desired appearance of the finished printed matter but they must be uniform throughout a particular stock.
An uncoated paper surface is not completely smooth but contains higher and lower areas since the thickness of the felted cellulose fibers varies from point to point. The magnitude of these variations in thickness is reduced by the smoothing effect of calendering. However, if the paper is again moistened with water, the cellulose fibers tend to swell and "spring back," increasing the magnitude of the variations. To create a smooth and level printing surface, the coating must fill in all of the low areas of the paper; while, to provide a uniform surface for ink reception, the coating must also cover the fibers in the high areas. When a paper is moistened by application of an aqueous coating, the magnitude of the surface irregularities is increased, and a larger amount of coating must be applied to create a uniform surface.
One commercial method employed to achieve the desired uniform surface is to follow application of the coating to the paper surface with a doctor blade. This application method usually results in even coatings; however, a particular problem with blade coaters is that any deformity in the blade edge can result in scratches in the coating surface. Scratches induced by doctor blades in the coating process are a primary reason for product rejection by quality control, which represents a major manufacturing cost. To an extent, a lubricant additive (such as calcium stearate) in the coating formulation can be employed to assist coating flow properties and will effect some "healing" of the coating surface scratches, thus preserving a uniform surface. The effectiveness of such lubricants, however, is limited by several factors, among which are the % solids in the coating formulation and the severity (i.e., depth and width) of the scratch. A high % solids coating formulation is desired by the manufacturer of coated papers for several reasons.
A high solids content (absent excessive scratching) produces a higher quality coating. Also, a reduction in coating formulation solvent content can reduce production costs significantly, not just in reduced formulation costs but the reduced drying time increases production rate and saves energy costs.
Therefore, an object of this invention is to provide an additive for paper coating formulations which improves the coating's ability to "heal" scratches produced upon application of the coating onto the paper. Another object of the invention is to provide an additive for paper coating formulations which permits the use of coating formulations of higher solids content. Also, an object of the present invention is to provide an additive for paper coating formulations which result in improved paper coatings.